Equilibrium - Result Question 82
####86. What is $[H^{+}]$in $mol / L$ of a solution that is $0.20 M$ in $CH_3 COONa$ and $0.10 M$ in $CH_3 COOH$ ? $K_a$ for $CH_3 COOH=1.8 \times 10^{-5}$.
[2010]
(a) $3.5 \times 10^{-4}$
(b) $1.1 \times 10^{-5}$
(c) $1.8 \times 10^{-5}$
(d) $9.0 \times 10^{-6}$
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Solution:
(d) $pH=p K_a+\log [\frac{\text{ Salt }}{\text{ Acid }}]$
$\log [H^{+}]=\log K_a-\log [\frac{\text{ Salt }}{\text{ Acid }}]$
$\log [H^{+}]=\log K_a+\log [\frac{\text{ Acid }}{\text{ Salt }}]$
$ \begin{aligned} {[H^{+}] } & =K_a[\frac{\text{ Acid }}{\text{ Salt }}] \\ & =1.8 \times 10^{-5} \times \frac{0.1}{0.2}=9 \times 10^{-6} M \end{aligned} $
